Chucks are most often used for the fixing, vis-a-vis a driveshaft, either of a bore bit or drill bit, or of a screwdriver end. A first possibility for fixing the cylindrical shank of a tool to the chuck consists in equipping the latter with a rack system which, with the aid of a key, tightens the jaws onto the tool before the machine is used. The rack provides rotational entrainment, with respect to the body, of a sleeve surrounding the latter, on the inner face of which is fixed a nut, the internal screw thread of which interacts with the external screw thread of the jaws. Although this solution gives satisfaction from a practical point of view, it exhibits the drawback of requiring the use of a key which is often mislaid by the user. This use of a key is all the more restrictive in so-called cordless drills, that is to say ones which are powered by batteries forming an integral part of the body of the machine which gives them very great mobility. These cordless machines or drills are generally equipped with key-type chucks for holding tools. This type of chuck allows use in both directions of rotation, and the passage of an axial screw for blocking the chuck on the spindle of the drill when the direction of rotation of the driveshaft tends to favor the unscrewing of the chuck vis-a-vis the driveshaft. Chucks do exist which allow satisfactory clamping of the tool without requiring the use of a key.
A first known solution consists in forming, at the rear end of the sleeve for rotational entrainment of the nut for actuating the jaws, several teeth, a ring mounted concentrically on the body being held axially away from the sleeve under the action of a spring including at least one tooth opposite the sleeve. This ring is mounted so that it can rotate freely around the body.
In practice, in order to clamp or release the tool, it is appropriate for the user to grasp hold of the ring and move it forward, so that the teeth in this ring come into mesh with the teeth in the sleeve, at the same time as he causes the rotation of the body of the chuck, from the driveshaft, in one direction or the other. A relative rotational movement of the antagonistic teeth of the ring and of the sleeve is produced, giving rise to a series of impacts promoting the tightening or loosening of the jaws, depending on the envisaged direction of rotation.
In another known chuck, the sleeve exhibits, at its front end, a part in the form of a radial collar from which there project several notches pointing toward the rear. Moreover, a ring including notches pointing forward, and intended to come into mesh with the notches which the sleeve includes is forcibly fitted over the nut for actuating the jaws.
A spring bearing on the body of the chuck holds the notches of the ring and of the sleeve, respectively, in mesh, as long as a certain load is not reached.
In practice, it is sufficient for the user, in order to tighten or loosen the jaws, manually to block the sleeve in terms of rotation and to actuate the driveshaft in one direction or the other. This results in a series of impacts exerted by the notches of the ring against the notches of the sleeve, promoting tightening or loosening, depending on the case.
However, these two known chucks have a complicated structure which leads to a high cost price and a very significant bulk.
Another solution is provided in document EP-A-0,300,375 which relates to a chuck using a very fine pitch thread for the jaws, and a series of balls located between the nut and the body, this latter characteristic, namely the interposition of the balls between the nut and the body, already being known in industrial chucks. The chuck described in this document exhibits the drawback of not possessing perfect non-loosenability characteristics, and of being very tricky to operate both for tightening and for loosening, accounting for the fineness of the pitch and, finally, of requiring a large outer diameter of the chuck in order to obtain good clamping.
Self-tightening chucks including an inner thrust ball bearing exhibit the drawback of not being able to be used in left-handed rotation, of not allowing the passage of a central screw for blocking the chuck onto the spindle of the machine, and of working loose under heavy vibrations during hammering works.
The devices described previously do not, however, give satisfaction in the case of high-performance machines which require mechanical locking which prevents the chuck from opening, especially during hammering works.